MLX71120 Datasheet, PDF (10/30 Pages) Melexis Microelectronic Systems – 300 to 930MHz FSK/FM/ASK Receiver

MLX71120
300 to 930MHz
FSK/FM/ASK Receiver
1.13 Data Filter
The data filter is formed by the operational amplifier OA1, two internal 100kΩ resistors and two external ca-
pacitors. It is implemented as a 2^nd order Sallen-Key filter. The low pass filter characteristic rejects noise at
higher frequencies and therefore leads to an increased sensitivity.
CF1
DF1
CF2
DF2
data filter
100k
100k
OA1
DF0
Fig. 5: Data filter
The filter’s pole locations can be set by the external capacitors CF1 and CF2. The cut-off frequency f_c has to
be adjusted according to the transmission data rate R. It should be set to approximately 1.5 times the fast-
est expected data rate. For a Butterworth filter characteristic, the data filter capacitors can be calculated as
follows.
R_RZ [kbit/s]
0.6
1.2
1.6
2.4
3.3
4.8
6.0
CF1 =
1
CF2 = ^CF1
PRELIMINARY 2 ⋅ π ⋅100k ⋅ fc
R_NRZ [kbit/s]
1.2
2.4
3.2
4.8
6.6
9.6
12
fc [kHz]
0.9
1.8
2.4
3.6
5
7.2
9
2
CF1 [pF]
2200
1200
1000
680
470
330
220
CF2 [pF]
1000
680
470
330
220
150
100
1.14 Data Slicer
The purpose of the data slicer is to convert the filtered data signal into a digital output. It can therefore be
considered as an analog-to-digital converter. This is done by using the operational amplifier OA2 as a com-
parator that compares the data filter output with a threshold voltage. The threshold voltage can be derived in
two different ways from the data signal.
SLCSEL
0
1
Description
Averaging detection mode
Peak detection mode
39010 71120
Rev. 005
Page 10 of 30
Data Sheet
Feb/08